WL teachers are always looking for new strategies to keep kids in the target language, especially when you let them loose for communicative activities. My go-to strategy is to make sure that all students have a piece of paper in their hands (even if the activity doesn't have a form or worksheet, have them hold a small piece of paper). As you move through the room listening to the conversations taking place between partners or groups, hold a hole puncher in your hand. If you hear English OR if students are not following other directions (such as not speaking in complete sentences, swapping papers without speaking, etc.), punch a hole in their paper. I usually assign one of my students to be an undercover "Policía", and she or he has a second hole puncher as well. At the end of the activity, each hole punch translates to some sort of a grade reduction--usually, I will count the activity as a Citizenship grade, and those students with one or more holes punched drop to Developing or Emerging for participation (within the Citizenship category). Depending on your grading system and how the particular assignment is being evaluated, it could mean the loss of one letter grade, -5 points, the addition of a writing assignment as a consequence, etc.
What strategies do YOU use to keep students in the target language during communicative activities?